This invention relates to a circuit for generating a bias voltage for another circuit which is integrated on a semiconductor substrate. The first-mentioned circuit comprises an oscillator for generating control pulses and at least one charge pump to which electrical pulses derived from the control pulses are applied. The charge pump comprises a series arrangement of a capacitance and a diode. The electrical pulses are applied to a first electrode of the capacitance, whose second electrode is connected to the diode associated with the capacitance. An output of the charge pump is connected to the substrate and the junction point of the capacitance and the diode of the charge pump is connected to the ground point of the integrated circuit via a channel of an insulated-gate switching transistor whose gate is connected to a control circuit which receives the control pulses.
Such a circuit is known from U.S. Pat. No 4,438,346. In the prior art circuit, the control electrode of the transistor, which connects the junction point of the capacitance and the diode of the charge pump to the ground point, is connected to a junction point of two series-arranged, diode-connected transistors which interconnect the ground point and a junction point carrying the negative substrate voltage. Hence, the control electrode is at a negative potential when there are no control pulses, thus causing the transistor to remain in the cut-off state if the voltage at the junction point in the charge pump decreases to a value which lies more than one threshold voltage of said transistor below ground potential. Thus, during a pumping cycle efficient use is made of the charge stored in the capacitance. However, in order to charge the capacitance, the negatively-biassed transistor must be rendered conductive. In said circuit this is achieved by means of control pulses which are applied to the control electrode of the transistor via a capacitor and which exceed the supply voltage.
For generating such control pulses, a relatively complex control circuit is needed in which the required voltage levels of the control pulses can be generated by means of bootstrap techniques.
However, the said U.S. Patent also describes steps, whereby the control pulses, generated by the relatively complex control circuit, are no longer needed. The control electrode of the switching transistor is connected to the ground point via the junction point of the capacitance and the diode of the charge pump. However, this circuit, which is known per se, has the disadvantage that the capacitance is charged to a maximum of V.sub.DD -2 V.sub.TH (V.sub.DD is the supply voltage and V.sub.TH is the threshold voltage of the field-effect transistors. The capacitance is usually formed by interconnecting the main electrodes of a field-effect transistor). However, at this low supply voltage the charge pump cannot pump much charge (or no charge at all if V.sub.DD &lt;2 V.sub.TH).